The retirement board may require any disability beneficiary who has not yet attained the age of sixty-five years to undergo a medical examination once each year. Such an examination shall be made (1) at the place of residence of the beneficiary or other place mutually agreed upon, (2) at the expense of the retirement system, and (3) by a physician, legally authorized to practice medicine under the laws of the state in which he or she practices, designated by the retirement board. If any disability beneficiary who has not yet attained the age of sixty-five years willfully refuses to submit to at least one such medical examination in any fiscal year, his or her allowance may be discontinued until his or her withdrawal of such refusal. If such refusal continues for one year, his or her rights in and to his or her disability retirement allowance may be revoked by the retirement board.